The present invention relates to a plasma CVD (Chemical Vapor Deposition) apparatus for preparation of a thin film used in various electronic devices such as an amorphous silicon solar cell, a microcrystalline solar cell, a thin film polycrystalline solar cell, a thin film semiconductor device, an optical sensor, and a semiconductor protective film.
Various plasma CVD apparatuses are used for preparation of an amorphous silicon (hereinafter referred to as xe2x80x9ca-Sixe2x80x9d) thin film, a microcrystalline thin film, a polycrystalline thin film, or a silicon nitride (hereinafter referred to as xe2x80x9cSiNxxe2x80x9d) thin film. The conventional plasma CVD apparatus can be classified typically into a type in which is used a ladder type electrode for discharge generation and another type in which are used plate electrodes arranged in parallel. The ladder type electrode includes, for example, a ladder antenna electrode and a ladder inductance electrode.
Japanese Patent Disclosure (Kokai) No. 4-236781 discloses a plasma CVD apparatus using a ladder type electrode of various shapes. FIG. 10 shows a typical example of the plasma CVD apparatus disclosed in JP ""781 quoted above. As shown in the drawing, a ladder type electrode 2 for discharge generation and a heater 3 for heating a substrate are arranged in parallel within a reaction vessel 1. A high frequency power having a frequency of, for example, 13.56 MHz is supplied from a high frequency power source 4 to the ladder type electrode 2 for discharge generation through an impedance matching device 5. As shown in FIG. 11, the ladder type electrode 2 for discharge generation is connected at one end to the high frequency power source 4 via the impedance matching device 5 and is also connected at the other end to a ground lead 7 and, thus, to the ground. Also, the reaction vessel 1 is connected to the ground.
The high frequency power supplied to the ladder type electrode 2 for discharge generation serves to generate a glow discharge plasma in a free space between the substrate heater 3, which is also connected to the ground together with the reaction vessel 1, and the ladder type electrode 2 for discharge generation. After generation of the glow discharge plasma, the high frequency power flows through the discharge space into the wall of the reaction vessel 1 and into the ground through the ground lead 7 connected to the ladder type electrode 2. A coaxial cable is used as the ground lead 7.
A mixed gas consisting of, for example, monosilane and hydrogen is supplied from a bomb (not shown) into the reaction vessel 1 through a reactant gas introducing pipe 8. The reactant gas introduced into the reaction vessel 1 is decomposed by a glow discharge plasma generated by the ladder electrode 2 for discharge generation so as to be deposited on a substrate 9 disposed on the heater 3 and heated to a predetermined temperature. On the other hand, the gas within the reaction vessel 1 is exhausted by a vacuum pump 11 through an exhaust pipe 10.
In preparing a thin film by using the apparatus described above, the inner space of the reaction vessel 1 is exhausted first by operating the vacuum pump 11, followed by introducing a mixed gas consisting of, for example, monosilane and hydrogen into the reaction vessel 1 through the reactant gas introducing pipe 8. In this step, the inner pressure of the reaction vessel 1 is maintained at 0.05 to 0.5 Torr. Under this condition, a high frequency power is supplied from the high frequency power source 4 to the ladder type electrode 2 for discharge generation so as to generate a glow discharge plasma. Therefore, the reactant gas is decomposed by the glow discharge plasma generated in the free space between the ladder type electrode 2 and the substrate heater 3 so as to generate Si-containing radicals such as SiH3, and SiH2. These radicals are attached to a surface of the substrate 9 so as to form an a-Si thin film.
FIG. 12 shows another type of the conventional plasma CVD apparatus in which are used plate electrodes arranged in parallel. As shown in the drawing, the apparatus comprises a reaction vessel 21. A high frequency electrode 22 and a substrate heater 23 are arranged in parallel within the reaction vessel 21. A high frequency having a frequency of, for example, 13.56 MHz is supplied from a high frequency power source 24 to the high frequency electrode 22 through an impedance matching device 25. The substrate heater 23 is connected to the reaction vessel 21. Also, the reaction vessel 21 is connected to the ground. It follows that the substrate heater 23 is indirectly connected to the ground to constitute a ground electrode, with the result that a glow discharge plasma is generated in the free space between the high frequency electrode 22 and the substrate heater 23.
A mixed gas consisting of, for example, monosilane and hydrogen is supplied from a bomb (not shown) into the reaction vessel 21 through a reactant gas introducing pipe 26. On the other hand, the gas within the reaction vessel 21 is exhausted by a vacuum pump 28 through an exhaust pipe 27. A substrate 29 is disposed on the substrate heater 23 so as to be heated to a predetermined temperature.
For forming a thin film by using the apparatus shown in FIG. 12, the inner space of the reaction vessel 21 is exhausted first by operating the vacuum pump 28, followed by introducing a mixed gas consisting of, for example, monosilane and hydrogen into the reaction vessel 21 through the reactant gas introducing pipe 26. In this step, the inner pressure of the reaction vessel 21 is maintained at 0.05 to 0.5 Torr. If a high frequency power is supplied from the high frequency power source 24 to the high frequency electrode 22, a glow discharge plasma is generated within the reaction vessel.
The monosilane gas contained in the mixed gas supplied through the reactant gas introducing pipe 26 into the reaction vessel 21 is decomposed by the glow discharge plasma generated in the free space between the high frequency electrode 22 and the substrate heater 23 so as to generate Si-containing radicals such as SiH3 and SiH2. These Si-containing radicals are attached to a surface of the substrate 29 so as to form an a-Si thin film.
However, any of the prior arts using a ladder type electrode and plate electrodes arranged in parallel gives rise to problems as described below.
(1) In the apparatus shown in FIG. 11, a reactant gas, e.g., SiH4, is decomposed by an electric field generated in the vicinity of the ladder type electrode 2 into Si, SiH, SiH2, SiH3, H, H2, etc. so as to form an a-Si film on the surface of the substrate 9. However, if the frequency of the high frequency power is increased from the present level of 13.56 MHz to 30 to 150 MHz in an attempt to increase the rate of forming the a-Si film, the electric field in the vicinity of the ladder type electrode fails to be distributed uniformly, leading to a markedly poor uniformity in the thickness of the formed a-Si film. FIG. 13 is a graph showing the relationship between the plasma power source frequency and the film thickness distribution in respect of a substrate having an area of 30 cmxc3x9730 cm. It should be noted that the size of the substrate which permits ensuring a uniformity in the film thickness distribution, i.e., deviation of xc2x110% from an average film thickness, is 5 cmxc3x975 cm to 20 cmxc3x9720 cm.
The reason why it is difficult to increase the frequency of the high frequency power source 4 in the apparatus using a ladder type electrode is as follows. Specifically, non-uniformity of impedance derived from the construction of the ladder type electrode is inherent in the apparatus shown in FIG. 10, with the result that a strong plasma light emission is localized, as shown in FIG. 14. For example, a strong plasma is generated in a peripheral portion alone of the ladder type electrode, and is not generated in a central portion. The difference in the plasma density between the peripheral portion and the central portion of the ladder type electrode is rendered prominent particularly where the frequency of the high frequency power source is increased to 60 MHz or more.
Under the circumstances, it is very difficult and considered substantially impossible to increase the film forming rate by increasing the frequency of the plasma power source when it comes to a large substrate required for improving the mass productivity and cost reduction. It should be noted that the film forming rate of a-Si is proportional to the square of the frequency of the plasma power source. Therefore, vigorous researches are being made in this technical field on the technology to increase the frequency of the plasma power source. However, a successful result has not yet been reported in the case of a large substrate.
(2) In the apparatus shown in FIG. 12, a reactant gas, e.g., SiH4, is decomposed by an electric field generated in the free space between the high frequency electrode 22 and the substrate heater 23 into Si, SiH, SiH2, SiH3, H, H2, etc. so as to form an a-Si film on the surface of the substrate 29. However, if the frequency of the high frequency power is increased from the present level of 13.56 MHz to 30 to 200 MHz in an attempt to increase the rate of forming the a-Si film, the electric field generated in the free space between the high frequency electrode 22 and the substrate heater 23 fails to be distributed uniformly, leading to a markedly poor uniformity in the thickness of the formed a-Si film. FIG. 13 is a graph showing the relationship between the plasma power source frequency and the film thickness distribution in respect of a substrate having an area of 30 cmxc3x9730 cm. It should be noted that the size of the substrate which permits ensuring a uniformity in the film thickness distribution, i.e., deviation of xc2x110% from an average film thickness, is 5 cmxc3x975 cm to 20 cmxc3x9720 cm.
The reason why it is difficult to increase the frequency of the high frequency power source 24 in the apparatus using plate electrodes arranged in parallel is as follows. Specifically, the peripheral portion and the central portion of the parallel plate type electrodes differ from each other in the electrical characteristics, with the result that a strong plasma is generated in the peripheral portions of the parallel electrodes 22 and 23 as shown in FIG. 15A, or a strong plasma is generated in the central portion alone of the parallel electrodes 22 and 23 as shown in FIG. 15B.
Under the circumstances, it is very difficult and considered substantially impossible to increase the film forming rate by increasing the frequency of the plasma power source when it comes to a large substrate required for improving the mass productivity and cost reduction. It should be noted that the film forming rate of a-Si is proportional to the square of the frequency of the plasma power source. Therefore, vigorous researches are being made in this technical field on the technology to increase the frequency of the plasma power source. However, a successful result has not yet been reported in the case of a large substrate.
An object of the present invention is to provide a plasma chemical vapor deposition apparatus, in which is used a power distributor for uniformly distributing a high frequency power to a ladder-shaped electrode through a power supply wire for vacuum, making it possible to obtain a film thickness distribution markedly superior to that obtained in the conventional apparatus.
Another object of the present invention is to provide a plasma chemical vapor deposition apparatus, comprising an impedance matching device which is connected at one end to a high frequency power source for supplying a high frequency power of 30 MHz to 200 MHz for a glow discharge generation to a ladder-shaped electrode for discharge generation and at the other end to the power distributor noted above so as to obtain a further improved film thickness distribution.
Still another object is to provide a plasma chemical vapor deposition apparatus, in which an impedance converter is interposed between a ladder-shaped electrode and a power distributor, at least two ladder-shaped electrodes are arranged on a plane parallel to a heater for heating a substrate, and the high frequency power generated from the power source is supplied to the ladder-shaped electrodes for discharge generation through the impedance matching device, the power distributor and a coaxial cable for vacuum, thereby making it possible to use the plasma chemical vapor deposition apparatus for forming a uniform a-Si thin film having such a large area as about 1 mxc3x972 m, a uniform microcrystalline silicon thin film having such a large area as about 1 mxc3x972 m, and a uniform polycrystalline silicon thin film having such a large area as about 1 mxc3x972 m.
According to one embodiment of the present invention, there is provided a plasma chemical vapor deposition apparatus for forming an amorphous thin film, a microcrystalline thin film or a polycrystalline thin film on a surface of a target substrate by utilizing a glow discharge generated by an electric power supplied from a power source, comprising:
a reaction vessel;
means for supplying a reactant gas into the reaction vessel;
discharge means for discharge a waste gas of the reactant gas out of the reaction vessel;
a ladder-shaped electrode for discharge generation arranged within the reaction vessel;
a power source for supplying a high frequency power of 30 MHz to 200 MHz to the ladder-shaped electrode for a glow discharge generation;
a heater for heating and supporting a target substrate, the heater being arranged within the reaction vessel in parallel to the ladder-shaped electrode for discharge generation; and
a power distributor for uniformly distributing a high frequency power to the ladder-shaped electrode for discharge generation through an electric wire for vacuum.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.